Every organism can be classified at 7 different levels of taxonomic heirarchy- Kingdom, Phylum/Division, Class, Order, Family, Genus and Species.
The kingdom is the largest and a broad category. Each successive group contains fewer organisms, but the organisms are more similar.
A genus is composed of many related species and these species have general characters which are common among them.
Species is the lowest taxonomic category and the characters are exclusive to that particular species.